Bioethanol derived from cellulosic biomass is unarguably the best candidate to be used as an alternative fuel in the light of the dwindling global fossil fuel supplies. Many parts of the world produce huge amounts of cellulosic biomass, much of which have never been reused or recycled. These biomass often end up as waste material that are incinerated, land-filled or left to fallow. The thought that if every scrap of cellulose biomass is converted to bioethanol is mind staggering, as the mass conversion to bioethanol could significantly help meet the global transportation fuel needs.
The present production of bioethanol from cellulosic biomass faces the problem of hydrolyzing recalcitrant cellulosic biomass to sugar in a quick and cost-effective manner. The current method for deconstructing cell-wall biopolymers into sugar building blocks takes the form of chemical and biological methods. The chemical methods are generally energy-intensive and often require harsh conditions, while the biological methods are dependent on the discovery and invention of effective genome-based cellulase-producing microorganisms.
The conventional method of producing bioethanol starts with the hydrolysis of cellulosic biomass to form glucose as described by the Arkanol process. In this process, concentrated sulphuric acid of 70% to 77% strength is added to the biomass. This was followed by an initial heating to 50° C., then dilution, and the final mixture is further heated to 100° C. for one hour. A gel is produced from this process which is pressed to release an acid-sugar mixture and the acid is separated from the sugar using a chromatographic column. The method appears to require a high input of energy.
It is commonly known in the art that the use of concentrated sulphuric acid often leads to charring of the sugar solution because concentrated sulphuric acid is a strong dehydrating agent. As a result, other methods such as the use of dilute sulphuric acid have also been developed. The hemicellulose portion of the biomass is hydrolyzed using 0.5% sulphuric acid at 190° C. and the cellulose portion is hydrolyzed using 0.4% sulphuric acid at 215° C. The solution is then neutralized and recovered. Pressures of up to 15 atm are applied to help hydrolyze the cellulosic biomass. This process appears to suffer from the need to have a high input of energy.
The step of the neutralization of sulphuric acid is done by using calcium hydroxide, calcium oxide or other calcium bearing material to give calcium sulphate, Ca2SO4.5H2O, or gypsum as a side product. This product is difficult to be separated out from the sugary solution. Besides, the large amount of calcium sulphate produced leads to disposal problems due to its limited usage.
There are a few patented technologies over the prior arts relating to the production process of cellulosic bioethanol. One of the patented technologies, U.S. Pat. No. 5,597,714, discloses a strong acid hydrolysis process of cellulosic and hemicellulosic materials. The acids used include phosphoric acid and sulfuric acid. Another U.S. Pat. No. 4,529,699 also relates to an acid hydrolysis of cellulosic materials using phosphoric acid, sulfuric acid, sulfurous acid or hydrochloric acid.
However, all these methods require high temperature and/or high pressure that are maintained over a long period. The process appears to be complicated, time-consuming and inconvenient, particularly in the process of extracting off the sugar produced.
The use of a superacid, dilute perchloric acid has also been disclosed in British Patent No. GB521884. This acid is used in the manufacture of cellulose ester, especially cellulose acetate from cellulosic material. Nevertheless, there is no technical guidance if the cellulose acetate can be converted to fermentable or simple sugar for bioethanol production.
U.S. Patent Publication No. 20080008783 disclosed uses alkali instead of acid to produce monosaccharides. The alkali used is concentrated ammonium hydroxide with or without anhydrous ammonia addition. Treatment of cellulosic material using base can destroy the lignin and hemicellulose which helps in saccharification. Apart from the complicated equipment set-up including pump, reactor, condenser and others, this process also requires high temperature and energy input.
There is also another process for treating lignocellulosic material as disclosed in U.S. Patent Publication No. 20040016525 that relates to the use of a condition in which the pH shall not be less than 8. After exposing the cellulosic material to this pH condition and steaming it at a first pressure, it is then discharged explosively to a second pressure before it can be used in the synthesis of bioethanol. Likewise, the process appears to be complicated, inconvenient, time- and energy-consuming.
The biological method focuses on the use of plants and microbes for the use of bioethanol production, such as the Trichoderma reesei (the cotton rot fungus) or Populus trichocarpa (a poplar). The attempts to deconstruct the cell-walls of biopolymers into sugar building blocks are presently beset with the high cost of enzymes, the quick denaturation of enzymes leading to slow conversion and the public's fear of genetically-engineered microorganisms that could be inadvertently released to the environment. Even the approach to hydrolysis of cellulosic materials using the combination of chemical and biological means faces over-riding factors such as costs and efficiency.
Therefore, it is desirable for the present invention to provide a method for producing sugar from cellulosic material which is more simple yet innovative to overcome some of the drawbacks of the prior arts.